1. Field of the Invention
This invention relates generally to a still picture transmitting apparatus and is more particularly directed to a video telephone which can transmit video data as well as audio data through general telephone lines.
2. Description of the Prior Art
When realizing a video telephone, if a video signal is transmitted as it is, expensive special telephone lines of a wide band are required. Therefore, except for an enterprise or the like using such a video telephone in an important conference, individuals cannot afford such a video telephone readily.
For this reason, a personal video telephone must use public telephone lines. A public telephone line, however, is used to transmit an audio signal and hence its transmission band ranges from 300 to 3400 Hz so that in this case, an image to be transmitted must be a still picture, and the video signal thereof is timebase-expanded for transmission.
FIG. 1 illustrates a transmission system of such a conventional video or television telephone. A video camera 1 produces a luminance signal Sy, and this signal Sy is supplied to an A/D (analog-to-digital) converter 2, in which it is converted into a digital signal. Then, the digital signal of one field period is written into a memory 3 and is also read out therefrom at a predetermined low speed. The thus read-out signal is supplied to a register 4. Then, the digital signal of one field period is written in a memory 3 and is also read out therefrom at a predetermined low speed. The converts the signal from a parallel signal to a serial signal. The serial signal therefrom is supplied to a MODEM 5, in which it is converted into an FSK signal of a frequency band lying within the transmission band of public telephone lines. This FSK signal is transmitted to a public telephone line 9.
Further, the luminance signal Sy is supplied to a synchronizing separating circuit 6 which extracts therefrom a vertical synchronizing pulse Pv and a horizontal synchronizing pulse Ph. These pulses Pv and Ph are supplied to respective circuits as timing signals. Also, the pulse Ph is supplied to a PLL (phase-locked loop) circuit 7 which derives an alternating signal having frequency 4fc four times as high as a color subcarrier frequency fc. The alternating signal therefrom is supplied to a frequency-dividing circuit 8, in which it is frequency-divided into pulses of predetermined frequencies. These frequency-divided pulses are supplied to the circuits 2 to 5 as clocks thereof.
Thus, according to such a video telephone, an image can be transmitted, by utilizing the public telephone lines, in the form of a black-and-white still picture.
Meanwhile, the number of picture elements in one frame, however, is expressed as EQU 525 horizontal lines.times.(55525.times.4.times.3) picture elements=367500 picture elements
Thus, if the bit number per picture element is assumed as 8 bits where a transmission rate is 2400 bps, the transmission time is expressed as follows. EQU 367500 picture elements.times.8 bits/2400 bps=1225 seconds
As a result, it take more than 20 minutes to transmit one still picture, and this cannot be done in practice.
Furthermore, the memory 3 needs a capacity of 8 bits.times.367.5 k addresses and this is also not practical.